Reduction of interference in thermionic valve circuits



April 23, 1936. P. w. WILLANS 2,038,879

v REDUCTION OF INTERFERENCE IN THERMIONIC VALVE CIRCUITS Filed May 26, I953 2 SheetS- -Sheei l I I IHTERME DIATE' AUDIO FR: 5 i 00 FReouEnu 22 AMPLIFIER AMPLIFIER LOCAL I Auplo FREQUENCY 59 v AMPLIFIER K 5/\8 I V f L F4 9 51} MIMI slaw //V/ f/V7' 01? A5759 MLL/AM W/LL ANS ATTOE/Vf)" q u r v $333M] 2 1%6. F WiLLANs 2,@s8,- 'i

REDUCTION OF INTERFERENCE IN THERMIONIC VALVE cIRcUIT Filed May 26, 1953 ZBheets-Sheet 2 38 INTERMEDIHTE 8 AND Aumo 39 FREQUENCY ,AMPLIFIER I d mhmm M I l l'hl lwl hl l l l l llHMI Patented Apr. 28, 1936 UNITED STATES PATENT OFFICE REDUCTION OF INTERFERENCE IN THERIVIIONIC VALVE CIRCUITS of Great Britain Application May 26, 1933, Serial No. 672,997

In Great Britain May 26, 1932 15 Claims.

The present invention relates to the reduction of interference in thermionic valve circuits.

It is known for various purposes to provide a tuned circuit common to the anode-cathode and grid-cathode circuits of a thermionic valve. For

example such a tuned circuit has been made resonant at or near the lower limit of a band of frequencies to be amplified for the purpose of rendering the amplification more nearly uniform over the band. In another known arrangement the potential differences developed across such a tuned circuit in a wireless receiver have been at the wanted frequency (that is to say the frequency of the carrier to be received) and have been passed on to the detector.

It has also been proposed to connect an impedance, such as a choke, in parallel with a condenser between the cathode of a thermionic amplifier valve and the negative terminal of the source of anode current supply, for the purpose of providing a desired frequency response characteristic. The condenser has been given a value such that its impedance for the lowest frequency at which the amplifier works is small in comparison with the impedance which it shunts. The resonant frequency of the impedance and condenser in parallel has therefore been below the operating range of the amplifier.

The present invention is concerned with the method of reducing the amplitude of an unwanted interfering oscillation according to which the effective amplification of a thermionic valve is reduced at the frequency or frequencies of the unwanted oscillations with the aid of means producing an anti-regenerative feed-back from the anode circuit to the grid circuit of the valve. The said means may take the form of a reactive impedance or a combination of reactive impedances common to the anode-cathode and grid-cathode circuits of the valve. The amplification will in this arrangement be least when the impedance of the circuit common to the anode and grid circuits is a maximum.

According to the present invention, a ther- 45 mionic valve circuit comprises a resonant circuit tunable to a wanted frequency, and a thermionic valve having means comprising a second resonant circuit common to its anode and control grid circuits and adapted to produce anti-regenerative feedback at an unwanted frequency from the anode circuit to the control grid circuit of said valve, the tuning controls of both said resonant circuits being mechanically coupled for simultaneous actuation.

55 In a superheterodyne receiver, where, for example, the intermediate frequency circuits are fixedly tuned, it may be unnecessary to make the second resonant circuit tunable.

The invention accordingly provides a superheterodyne receiver in which there is employed a thermionic valve having in common with its anode-cathode and grid-cathode circuits a reso-' nant circuit comprising an impedance element or combination of impedance elements, the impedance of which is much higher atone or more unwanted frequencies than at a wanted frequency.

In the case of a superheterodyne wireless receiver, if it is desired to receive a modulated carrier of frequency 1000 kilocycles and if the intermediate frequency amplifier is tuned to 100 kilocycles, the local oscillator may be tuned to 900 kilocycles. If there should be an unwanted oscillation at a frequency close to 800 kilocycles, or if there should be a harmonic of a strong unwant ed'station at a frequency of 800 kilocycles, it will cause second channel interference by beating with the local oscillations to produce the intermediate frequency. According to a feature of the invention, therefore, in a superheterodyne receiver as set out in the preceding paragraph, said resonant circuit is tuned or made tunable to a frequency or band of frequencies at or within which second channel interference may occur.

A further application of the invention is to the purification of the oscillations generated by a beat oscillator. In generating oscillations of relatively low frequency, for example audio frequency, by beating together two oscillations of relatively high frequency and rectifying the resulting beats, it is known that if one of the two According to this invention, a beat oscillator comprises two sources of oscillations of different frequencies, and a detector-valve having means comprising a resonant circuit common to its anode-cathode and grid-cathode circuits for producing anti-regenerative feed-back from the anode circuit to the grid circuit of said detector valve at one or more of the harmonic frequencies of one of said sources of oscillations.

' Further, the present invention may be applied to wireless receivers adapted to operate over two or more different frequency ranges; in such receivers, it may be found that oscillations within the higher frequency range tend to break through and cause interference when the re-' ceiver is operated over the lower frequency range.

According to a feature of the present invention, a wireless receiver of this type comprises a tunable input circuit and switching means for changing the frequency range over which said circuit is tunable, and a thermionic valve having means comprising a resonant circuit common to its anode-cathode and grid-cathode circuits for producing anti-regenerative feed-back from the anode circuit to the grid circuit of said valve, over the whole or a part of one of the frequency ranges to which said input circuit is tunable.

The resonant circuit maybe thrown into and out of circuit automatically when the rangechanging switching means are operative. Itmay, if desired, take the form of a simple inductance.

The invention will be described byway of example with reference to the accompanying drawings, in which Fig. 1 is a circuit diagram of a superheterodyne receiver,

Fig. 2 is a further circuit diagram of a superheterodyne receiver,

Fig. 3'is a circuit diagram of a receiver for operation over two frequency ranges,

Figs. 4 and 5 show two alternative resonant circuits for use in association with the receiver of Fig. 3, and v V Fig. 6 shows an audio-frequency beat oscillator.

Referring to Fig. 1, an input'transformer comprises a primary winding I, connected in a suitable antenna-earth circuit, and inductively coupled to a secondary winding 2 which is shunted by a variable condenser 3, and is connected between the control grid of a screened grid first detector valve 4, and earth. The screen grid 5 of the valve 4 is connected through a coil 6 to a point in a source I of high tension current, and the coil 6 is coupled by means of a coil 8 to a source 9 of local oscillations. The frequency of the oscillations generated by the source 9 is determined by a variable condenser I0. The anode II of the valve 4 is connected to the source I of high tension current through a parallel resonant circuit I2 tuned to the desired intermediate frequency.

ed frequency f1, and the local oscillator 9 is generating oscillations of frequency fiifz, the circuit I6, I! may be tuned to a frequency f1i2f2, that is, the frequency of the second channel or imagefrequency interference corresponding to thefrequency ii. If the tuning of the circuit I6, I1 is sharp, there will be substantially no negative or anti-regenerative feed-back at the frequency f1, while the amplification of the valve 4 at the frequency 'f1i2f2 will be considerably reduced due to negativefeed back at this frequency. v 7

If an impedance is arranged in the anode circuit of a'valve of magnification factor m, it can be shown that the effect of making this impedance'common to the grid circuit is to increase its effective value in the anode circuit in the ratio lzl-i-m. The circuit I6, I! in the circuit described above is therefore (m-l-l) times more efiicient as a rejector of its resonant frequency than if it were connected in series in the input circuit, or in any other equivalent position.

The intermediate frequency circuit I2 is mutually coupled to .a similar parallel resonant circuit I8 which is in the input circuit of an intermediate frequency amplifier I9. The latter feeds a second detector valve 20 which is coupled to a low-frequency amplifier ZI having a pair of output terminals 22. Between the cathode 23 of the second detector valve 20 and the negative terminal of a source 24 which supplies the anode,

valve 20 at this frequency. The inductance coil 25 is arranged to have an impedance to the third and higher harmonics of the intermediate frequency which is considerably higher than its im pedance at the intermediate frequency itself; anti-regenerative feed-back due to the inductance coil 25 therefore serves to reduce interference due to the third and higher harmonics of the intermediate frequency.

It may be observed here that in the interests of efliciency the impedance of the resonant feedback circuit at the undesired frequency or frequencies should be as high as possible compared with the impedance of the valve with which it is associated. Subject to this consideration, the in vention is not primarily concerned with the nature of the resonant feed-back circuit, and as has been mentioned, a simple inductance coil may in some cases constitute a suitable circuit. Broadly, any resonant circuit is suitable for use as a feed-back circuit which offers an impedance to one or more unwanted frequencies which is high compared with the impedance offered to a wanted frequency. Where the unwanted frequency is particularly close to the wanted frequency, it may be desirable to employ a complex feedback circuit which acts as a rejector at the unwanted frequency, and at the same time acts. as. an acceptor to the wanted frequency. r

The superheterodyne receiver described with reference to Fig. 1 is of the type in which the antenna is coupled to the first detector without the interposition of a signal frequency amplifier stage, second channel interference being particularly manifest in such a receiver. Fig. 2 is a circuit diagram of a receiver having one stage of signal frequency amplification, and like parts in this figure are given the same references as in Fig. 1, with a dash suffix.

The primary winding I of the inputtransformer is coupled to an input circuit 2', 3 which is connected between the control grid of a screened grid signal frequency amplifier valve 28, r

and earth. Between'the anode 29 of this valve and the source I of high tension current is connected the primary winding 30 of a high frequency transformer, the secondary winding 3i of which is connected in parallel with a variable condenser 32 between the control grid of a first detector valve 4 and earth.

Between the cathode 33 of the valve 28 and earth is connected a biassing resistance 34 shunted by a bypass condenser 35, and, in series, a parallel resonant circuit comprising an inductance coil 36 in parallel with a variable condenser 31. The broken line indicates that the variable condensers 3', 31, 32, I1 and H1 are ganged for simultaneous actuation. The intermediate frequency circuit I8 is in the input circuit of an intermediate frequency amplifier which is coupled to a second detector feeding, in turn, a low frequency amplifier. The intermediate frequency amplifier, second detector and low-frequency amplifier are indicated at 38; the low-frequency output of the receiver is derived from a pair of terminals 39.

The parallel resonant circuit 36, 31 is made resonant at the frequency of second channel interference corresponding to the signal frequency to which the input circuit 2', 3' is tuned, and produces anti-regenerative feed-back at this frequency from the anode circuit to the grid circuit of the high-frequency amplifier valve 28. The circuit l6, l1 may be made resonant at the same frequency, or may be arranged to reduce interference due to other causes. For example, this circuit may be tuned to the second harmonic of the frequency generated by the source 9' of local oscillations, or it may be fixedly tuned to the second harmonic of the intermediate frequency. In'either case it may be replaced by a low-pass filter circuit tuned or tunable to reject all harmonics of the intermediate frequency, or the frequency of the local source.

It is to be understood that many modifications in the circuits of the receivers above described may be made within the scope of this invention. For example, it is not essential to couple the local source of oscillations to the screen grid circuit of the first detector valve; if desired, the local source may be coupled to an inductance coil forming a part of the feed-back circuit.

Referring now to Fig. 3, which is a circuit diagram of a receiver adapted for operation over two frequency ranges, an input transformer comprises a primary winding in two sections 40 and 4|. The primary winding is coupled to a secondary winding 42 and 43 which is shunted by a variable condenser 44 and connected between the control grid of a screened grid high frequency amplifier valve 45 and earth. The anode 46 of the valve 45 is connected to a source 41 of high tension current through the primary winding 48, 49 of a high frequency transformer, the second ary winding 50, 5! of which is shunted by a variable condenser 52 and is connected between the control grid 53 of a detector valve 54 and earth through a grid condenser 55. The control grid 53 is connected to earth through a grid leak 56. The screening grid 5! of the valve 45 is connected to a point in the source 41, and the detector valve 54 is coupled to a low frequency amplifier 58 having a pair of output terminals 59.

The sections 4!, 43, 49 and 5| of the input and intervalve transformers are provided with shortcircuiting switches S1, S2, S3 and S4, these switches being closed when operating on the higher frequency band and open when operating on the lower frequency band. All four switches may be operated by a single control. It is found that by reason, among other things, of stray capaci ties such as those between the switch contacts, the receiver may be responsive to a higher frequency signal when connected for operation over the lower frequency range, that is, when the switches are open. The higher frequency signal may tend to break through and cause interference with a programme within the lower frequency range.

' The cathode B0 of the valve 45 is connected to earth through a, biassing resistance 6! shunted by a by-pass condenser 62, and, in series, a resonant circuit comprising an inductance coil 63 in parallel with a condenser 64 which may be made variable. The circuit 33, 64 is made resonant to a frequency at which break through is experienced, and by producing negative feedback at this frequency, tends to reduce the amplification of the valve 45 at this undesired frequency. A switch S3 is provided to short-circuit the circuit 63, 64 when the switches S1, S2, S4

and'ss are closed and the receiver is operating on the higher frequency range. The switch S3 may be ganged with the switches S1, S2, S3 and S4 for simultaneous operation.

Reference will now be made to Figs. 4 and 5 which show two modifications of the receiver shown in Fig. 3. Certain of the elements of Fig. 3 are omitted from these figures, and elements which re-occur are given dash sufiixes in Fig. 4

and double-dash suffixes in Fig. 5. In Fig. 4, a

second resonant circuit comprising an inductance coil 65 in parallel with a fixed condenser 66 is connected in series with the resonant circuit B3, 64' between the cathode of the valve 45 and earth. The circuits 63, 64 and 65, are made resonant at two frequencies at which break through occurs; more than two resonant circuits tuned to further break through frequencies may be employed if desired.

In Fig. 5, the cathode 63 of the valve 45" is connected to earth through a low-pass filter comprising an inductance coil 63" shunted by a condenser 61 in series with an inductance coil 68, the latter being shunted by a terminating resistance 69. The filter is designed to pass the frequencies of the lower frequency range while presenting a high and substantially constant impedance to the frequencies of the higher range. The filter described above is given purely by way of example, and many other forms of impedance networks may be employed within the scope of the invention.

Referring now to Fig. 6, which shows a beat type audio-frequency generator, the triode valve H1 is arranged to produce oscillations of a fixed frequency, while the triode valve H is arranged to produce oscillations at'a frequency which can be varied by adjustment of the condenser 12. The oscillations produced bythe valve in are applied to the control grid 73 of a screen grid buifer valve 74, the anode E5 of which is connected to a high tension source 16 through a coupling coil TI. The coil 11 and the oscillator valve H are coupled to a coil'18 which is connected to the control grid 19 of a square law rectifying valve 83, and, through a biassing battery 8 l to earth.

The'anode 82 of the oscillator valve 15 is connected to a high tension source 83 through a parallel resonant circuit comprising an inductance coil 84 in parallel with a fixed condenser 85; Connected to the cathode 83 of the valve I0, and to the'control grid 87 through a grid condenser 88, is a coil 89 coupled to the coil 84. A grid leak 99 is'connected'between the grid 8'! and the cathode 85,and a tapping point on the coil 89 is connected to the grid 13 of the buffer valve 74.

The oscillator valve H is associated with similar circuits. resonant circuit comprising a coil 92 and the Its anode 9|. is connected through a condenser 12 to the source [6. The coil 92 is coupled to a grid coil 93, and agrid condenser 94 and a grid leak 95 are provided.

The anode 96 of the rectifier is connected to ahigh tension source 91 through a high frequency filter comprising a choke 98 and two condensers 9 9, and the primary winding of a low frequency transformer I00, the secondary winding of which is associated with a pair of output terminals MI, The negative terminals of the high tension sources 91, "I6 and 83 are earthed.

In a beat oscillator of this type it is found that a pure beat oscillation is obtained when the 'wave form of one of the two beating oscillations is pure, that is to say, undistorted by the presence of harmonics. The cathode I02 of the valve 80 is accordingly connected to earth through two circuits I03 and I04 each designed to reject a harmonic of the oscillation generated 'by the valve I0. The circuit I03 comprises a coil I05 in series with a fixed condenser I06, the series circuit being shunted by a fixed condenser I01. The series circuit I05, I06 is made resonant at the fundamental frequency, and the whole circuit is made to present a low impedance to the fundamental frequency of the oscillator I0, and a high impedance to. the second harmonic. The circuit I04 similarly comprises a coil I08, and condensers I09 and II 0, and is arranged to present a low impedance tothe fundamental while presenting a high impedance to the third or another harmonic. If desired, the circuits I03, I04 may be replaced by a single impedance network designed to pass the fundamental and reject a number of harmonics.

I claim:

1. Apparatus comprising a thermionic valve having an input circuit and an output circuit, a resonant circuit connected to said input circuit and being tunable over 'a range of desired frequencies and a second resonant circuit, coniprising an inductance coil in parallel with a condenser, common. to said input and output circuits and being tunable simultaneously with the first resonant circuit, said second resonant circuit beingtuned to an undesired frequency within said range and arranged to produce feed-back in antiregnerative sense at said undesired frequency from said output circuit to said input circuit whereby currents of said undesired frequency are prevented in said output circuit.

2. Apparatus comprising a. thermionic valve having an input circuit and an output circuit, an inductance coil associated electrically with one of said circuits, a variable condenser forming with said inductance coil a circuit tuned to a desired frequency and having an'adjustable control element, a second inductancecoil arranged to be common to said input and output circuits, a second variable condenser connected in parallel with said second inductance and having an adjustable control ,element, said second inductance coil and condenser'constituting a circuit resonant to an undesired frequency, and means 'mechanically coupling said control elements whereby said condensers can be simultaneously adjusted and currents of said undesired frequency substantially eliminated from said output circuit.

3. A superheterodyne radio receiver comprising a thermionic valve having an input circuit and an output circuit, an inductance coil associated electrically with one of said circuits, a variable condenser forming with said inductance coil a circuit tunedto a desired signal frequency and having an adjustable control element, a second inducta source of local oscillations, a thermionic valve coupled to said source and having an input circuit and an output circuit and a resonant circuit common to said input and output circuits, said resonant circuit being tuned to a harmonic of the oscillations generated by said source and being arranged to produce feed-back in antiregenerative sense from said output circuit to said input circuit at said harmonic frequency.

5. A superheterodyne receiver of modulated carrier waves comprising a source of local oscillations adapted to beat with said carrier waves to produce oscillations of a beat frequency, a thermionic valve having an input circuit and an output circuit, means coupling said oscillation source to said input circuit and a resonant circuit common to said input and output circuits, said resonant circuit being tuned to a harmonic of said beat frequency. 7

6. A superheterodyne radio receiver comprising a source of local oscillations,'a thermionic valve coupled to said source and having an input circuit and an output circuit and a rejector circuit comprising a plurality of impedance elements common to said input and output circuits, said rejector circuit having, at a plurality of frequencies which are harmonics of the frequency of the oscillations generated by said. source, impedance values which are high compared with the impedance thereof at a desired frequency.

'7. A superheterodyne radio receiver comprising a thermionic valve having an input circuit and an output circuit, and a rejector circuit comprising an inductance coil, a'second inductance coilin series with said coil and a condenser arranged in parallel with said second inductance coil, common to said input and output circuits, said rejector circuit having much higher impedance values for harmonics of the intermediate frequency than for currents of the intermediate frequency.

8. A superheterodyne receiver for receiving modulated carrier oscillations comprising an input circuit, a source of local oscillations, means for applying said local oscillations to said input circuit, control means for varying the frequency of said local oscillations, an output circuit electrically associated with said input circuit, a rejector circuit common to said inputand output circuits andarranged to reduce second channel interference in said output circuit, means for tuning said rejector circuit and means serving to couple said control and tuning means mechanically together for simultaneous operation.

'9. A beat oscillator comprising twosources of oscillations of different fundamental frequencies,

a detector valve having an' input circuit and an a detector valve having an input circuit and an output circuit, means for applying the oscillations from said sources to said detector valve, and a. resonant circuit tuned to a harmonic of one of said fundamental frequencies and comprising a combination of impedance elements common to said input and output circuits arranged to produce feed-back in anti-regenerative sense from said output circuit to said input circuit at a-plurality of frequencies corresponding to harmonics of the frequency of the oscillations generated by one of said sources.

11. A heat oscillator comprising two sources of oscillations of different fundamental frequencies, a detector valve having an input circuit and an output circuit, means for applying the oscillations from said sources to said detector valve, and a resonant circuit comprising a combination of impedance elements constituting an acceptor circuit for the fundamental frequency of the oscillations generated by one of said sources, and a rejector circuit for a harmonic of that frequency, common to said input and output circuits arranged to produce feed-back in anti-regenerative sense from said output circuit to said input circuit at said harmonic frequency.

12. A radio receiver comprising a thermionic valve having an input circuit and an output circuit, arranged for operation over a plurality of frequency ranges, switching means arranged to change the range over which said circuits are operable, a resonant circuit common to the input and output circuits of said valve arranged to produce feed-back in anti-regenerative sense from said output circuit to said input circuit at a frequency within one frequency range when said input and output circuits are connected for operation over another of said frequency ranges.

13. A radio receiver comprising a thermionic valve having an input circuit and an output circuit, arranged for operation over a plurality of frequency ranges, switching means arranged to change the range over which said circuits are operable, a resonant circuit comprising a combination of impedance elements constituting a lowpass filter arranged to offer an impedance to frequencies within a higher frequency range which is high compared with the impedance oifered to frequencies within a lower frequency range common to the input and output circuits of said valve and arranged to produce feed-back in antiregenerative sense from said output circuit to said input circuit over a band of frequencies within said higher frequency range when said input and output circuits are connected for operation over said lower frequency range.

14. A radio receiver comprising a thermionic valve having an input circuit and an output circuit, adapted for operation over a plurality of frequency ranges, switching means arranged to change the range over which said circuits are operable, a resonant circuit common to the input and output circuits of said valve arranged to produce feed-back in anti-regenerative sense from said output circuit to said input circuit at a frequency within one frequency range when said input and output circuits are connected for operation over another frequency range, and switching means mechanically coupled to said range changing switching means for short-circuiting said resonant circuit when said range changing switching means are operated to secure operation of said receiver over the higher of said frequency ranges.

15. A radio receiver comprising a thermionic valve having an input circuit and an output circuit arranged for operation over a plurality of frequency ranges, a plurality of switches connected to said circuits to change the range over which said circuits are operable, a resonant circuit consisting of a coil connected in parallel with a capacitor common to the said input and output circuits and arranged to product feed-back in antiregenerative sense from said output to said input circuit at a frequency within one frequency range when said input and output circuits are connected for operation over another of said frequency ranges and a single control means connected to operate said switches.

PETER WILLIAM WILLANS. 

